A specialized Map implementation for use with enum type keys. All
of the keys in an enum map must come from a single enum type that is
specified, explicitly or implicitly, when the map is created. Enum maps
are represented internally as arrays. This representation is extremely
compact and efficient.

Enum maps are maintained in the natural order of their keys
(the order in which the enum constants are declared). This is reflected
in the iterators returned by the collections views (keySet(),
entrySet(), and values()).

Iterators returned by the collection views are weakly consistent:
they will never throw ConcurrentModificationException and they may
or may not show the effects of any modifications to the map that occur while
the iteration is in progress.

Null keys are not permitted. Attempts to insert a null key will
throw NullPointerException. Attempts to test for the
presence of a null key or to remove one will, however, function properly.
Null values are permitted.

Like most collection implementations EnumMap is not
synchronized. If multiple threads access an enum map concurrently, and at
least one of the threads modifies the map, it should be synchronized
externally. This is typically accomplished by synchronizing on some
object that naturally encapsulates the enum map. If no such object exists,
the map should be "wrapped" using the Collections.synchronizedMap(java.util.Map<K, V>)
method. This is best done at creation time, to prevent accidental
unsynchronized access:

EnumMap

Creates an enum map initialized from the specified map. If the
specified map is an EnumMap instance, this constructor behaves
identically to EnumMap(EnumMap). Otherwise, the specified map
must contain at least one mapping (in order to determine the new
enum map's key type).

get

Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key
k to a value v such that (key == k),
then this method returns v; otherwise it returns
null. (There can be at most one such mapping.)

A return value of null does not necessarily
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to null.
The containsKey operation may be used to
distinguish these two cases.

clear

keySet

Returns a Set view of the keys contained in this map.
The returned set obeys the general contract outlined in
Map.keySet(). The set's iterator will return the keys
in their natural order (the order in which the enum constants
are declared).

values

Returns a Collection view of the values contained in this map.
The returned collection obeys the general contract outlined in
Map.values(). The collection's iterator will return the
values in the order their corresponding keys appear in map,
which is their natural order (the order in which the enum constants
are declared).

entrySet

Returns a Set view of the mappings contained in this map.
The returned set obeys the general contract outlined in
Map.keySet(). The set's iterator will return the
mappings in the order their keys appear in map, which is their
natural order (the order in which the enum constants are declared).